SinglyLinkedListNode* head2) {
    if(!head1)
    return head2;
    if(!head2)
    return head1;
else if(head1->data<=head2->data){
    head1->next = mergeLists(head1->next, head2);
    return head1;
    }
else{
    head2->next = mergeLists(head1, head2->next);
    return head2;
}
}